Bipolar transistor switch design is a tradeoff between ON state power dissipation, base current drive, and fast switching speed. To obtain low dissipation, the power transistor must be driven heavily into saturation to provide minimum ON state power dissipation, i.e. low forward voltage drop. This requires large base drive circuitry. Additonally, this results in slow switching speed at turn-off.
In prior applications where the bipolar power transistor may be subject to large overload currents for short periods of time, turn-off may be achieved quickly, but the turn-off dissipation is extremely high due to the greatly increased voltage. Upon occurence of an overload current, the bipolar power transistor will conduct only the amount of collector current which is enabled by the level of base drive applied. The transistor does not pass the remaining overload current, and thus voltage across the transistor increases. This increasing voltage, with constant collector current, carries the transistor into its active region, which in turn enables fast turn-off.
In these prior circuit applications, the voltage across the bipolar power transistor is allowed to increase until the short circuit protective circuitry extinguishes the overload current. The voltage across the transistor thus attains a very high level which in combination with the constant maximum collector current determines the power dissipation.
In other approaches high levels of base drive are always supplied to guarantee transistor saturation, which results in excessive base circuit dissipation, a large base drive power supply, and slow switching speed.
Other approaches regulate base drive current in response to emitter current to reduce power dissipation, but the optimizing of turn-off time and ON state dissipation is not maintained.